Topic: Culture

Last updated: April 23, 2019

The identification of bacteria is a careful and systematic process that uses many different techniques to narrow down the types of bacteria that are present in an unknown bacterial culture. It produces benefits for many aspects of the research of microorganisms and helps physicians correctly treat patients. Multiple tests were performed to provide the fermentation abilities, presence of certain enzymes, and certain biochemical reactions. Qualitative observations were made on the tests, which were compared to unknown bacteria identification key to aid with the identification process. Abstract
The identification of bacteria is important in the medical settings. The bacteria were received from the instructor in a culture tube as Unknown #1 and Unknown #3. The bacteria were then examined with Gram-stain, differential media, and selective media. Unknown #1 retained the crystal violet stain with a coccus shape while Unknown #3 retained the safranin stain with a bacillus shape. Unknown #1 grew on Mannitol Salt Agar (MSA), Phenylethyl alcohol agar (PEA), blood agar (BAP), starch, was DNase positive, and did not grow on Eosin methylene blue agar (EMB), Hektoen entereic agar (HEK), endo agar, or MacConkey agar. Unknown #3 exhibited the reverse phenotype, showing no growth in mannitol salt agar, phenylethyl alcohol agar, while growing on blood agar, starch, DNase, Eosin methylene blue agar, Hektoen enteric agar, endo agar, and MacConkey agar. In combination with the biochemical tests, Unknown #1 was identified as a Gram-positive coccus, most likely Staphylococcus aureus, and Unknown #3 was identified as a Gram-negative bacillus, most likely Proteus mirabilis.

Bacteria have long had an impact on human lives, even before they were first discovered. Depending on the situation, they can have a beneficial or detrimental impact on the host. It is essential to have the ability to identify and understand more about each bacterial strain and its mechanism of pathogenesis. A number of techniques have been developed in order to differentiate between the different types of bacteria and evaluate the different enzymes each strain produces. By giving each strain a specific phenotype, it should be possible to quickly delineate between completely different strains of bacteria. These techniques that are so heavily relied upon are used concurrently with more modern molecular techniques, such as 16S ribosomal DNA sequencing to get the most accurate assessment of genus and species.
The two main classes of bacteria are categorized as Gram-positive and Gram-negative bacteria. The main difference between the two types is the composition of the cell wall. Gram-positive bacteria have cell wall that has a thick layer of peptidoglycan, with a thin layer of lipopolysaccharide (LPS) surrounding the peptidoglycan layer. The peptidoglycan layer surrounds the cell membrane of Gram-positive bacteria. Peptidoglycan is a molecule that is composed of the sugars, N-acetylglucosamine and N-acetylmuramic acid, and amino acids, namely L-alanine and D-alanine. Peptidoglycan’s role is to provide cell rigidity, strength, and protection against osmotic lysis. Gram-negative bacteria have a thinner cell wall with multiple layers: a thin peptidoglycan layer in-between an inner cytoplasmic membrane and an outer bacterial membrane. LPS is a molecule that is comprised of a lipid molecule and a polysaccharide, providing the bacteria protection against toxic molecules and allows bacteria to evade some immune responses.

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